1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting an aperiodic sounding reference signal in a wireless communication system.
2. Related Art
In wireless communication systems, it is necessary to estimate an uplink channel or a downlink channel for the purpose of the transmission and reception of data, the acquisition of system synchronization, and the feedback of channel information. In wireless communication system environments, fading is generated because of multi-path time latency. A process of restoring a transmit signal by compensating for the distortion of the signal resulting from a sudden change in the environment due to such fading is referred to as channel estimation. It is also necessary to measure the state of a channel for a cell to which a user equipment belongs or other cells. To estimate a channel or measure the state of a channel, a reference signal (RS) which is known to both a transmitter and a receiver can be used.
A subcarrier used to transmit the reference signal is referred to as a reference signal subcarrier, and a subcarrier used to transmit data is referred to as a data subcarrier. In an OFDM system, a method of assigning the reference signal includes a method of assigning the reference signal to all the subcarriers and a method of assigning the reference signal between data subcarriers. The method of assigning the reference signal to all the subcarriers is performed using a signal including only the reference signal, such as a preamble signal, in order to obtain the throughput of channel estimation. If this method is used, the performance of channel estimation can be improved as compared with the method of assigning the reference signal between data subcarriers because the density of reference signals is in general high. However, since the amount of transmitted data is small in the method of assigning the reference signal to all the subcarriers, the method of assigning the reference signal between data subcarriers is used in order to increase the amount of transmitted data. If the method of assigning the reference signal between data subcarriers is used, the performance of channel estimation can be deteriorated because the density of reference signals is low. Accordingly, the reference signals should be properly arranged in order to minimize such deterioration.
A receiver can estimate a channel by separating information about a reference signal from a received signal because it knows the information about a reference signal and can accurately estimate data, transmitted by a transmit stage, by compensating for an estimated channel value. Assuming that the reference signal transmitted by the transmitter is p, channel information experienced by the reference signal during transmission is h, thermal noise occurring in the receiver is n, and the signal received by the receiver is y, it can result in y=h·p+n. Here, since the receiver already knows the reference signal p, it can estimate a channel information value ĥ using Equation 1 in the case in which a Least Square (LS) method is used.ĥ=y/p=h+n/p=h+{circumflex over (n)}  <Equation 1>
The accuracy of the channel estimation value ĥ estimated using the reference signal p is determined by the value {circumflex over (n)}. To accurately estimate the value h, the value {circumflex over (n)} must converge on 0. To this end, the influence of the value {circumflex over (n)} has to be minimized by estimating a channel using a large number of reference signals. A variety of algorithms for a better channel estimation performance may exist.
An uplink reference signal can be classified into a demodulation reference signal (DMRS) and a sounding reference signal (SRS). The DMRS is a reference signal used for channel estimation to demodulate a received signal. The DMRS can be associated with PUSCH or PUCCH transmission. The SRS is a reference signal transmitted for uplink scheduling by a user equipment to a base station. The base station estimates an uplink channel by using the received SRS, and the estimated uplink channel is used in uplink scheduling. Transmission of the SRS may be referred to the section 5.5.3 of 3GPP TS 36.211 V9.1.0 (2010-03), and an operation of a user equipment in an SRS transmission process may be referred to the section 8.2 of 3GPP TS 36.213 V9.1.0 (2010-03).
A SRS may be transmitted periodically or may be transmitted aperiodically by being triggered by a base station when the base station needs to transmit the SRS. A method for efficiently transmitting an aperiodic SRS is required.